Volcanoes, archaeology and the secrets of Roman concrete

Geophysical processes have shaped Pozzuoli, Italy, like few other places in the world. Stanford students applied modern tools to understand those links and what it means to live with natural hazards as both threat and inspiration.

High above Italy’s Tyrrhenian Sea, off the north coast of Sicily, 13 students sit atop Stromboli Volcano as it erupts. Ash falls on their shoulders and ping-ping-pings their helmets. The ground beneath their feet trembles.

The journey to Stromboli had begun the day before in Vanorio’s hometown, Pozzuoli, a colorful port city founded by the Greeks and later occupied by the Romans, at the center of a volcanic caldera, or depression, known as Campi Flegrei. Vanorio, the 13 students and two teaching associates boarded a hydrofoil in Naples and sailed south across the deep blue water of the Tyrrhenian for nearly four quiet hours before catching sight of smoke, steam and gases puffing from Stromboli’s cone.

During their two-week trip, students visited two volcanoes in Italy and local towns shaped by their proximity. (Image credit: Yvonne Tang)

Reaching the top would prove more arduous – a five-hour climb up steep slopes of ash and rock. Sedimentologist Nora Nieminski, a postdoctoral researcher at Stanford Earth and a guest instructor on the trip, sprinted ahead to shoot drone footage that she would later help the students manipulate to create 3D models of the volcano. But the rest of the group walked without hurry. Halfway to the top, they stopped to rest near a dark scar on the volcano’s northern flank known as the Sciara del Fuoco, where the volcano has collapsed on itself.

Dionne Thomas, ’20, a student on the trip who is majoring in chemical engineering, remembers smelling dirt and ash, seeing the Tyrrhenian Sea reflecting the sky’s late-afternoon wash of orange and blue, and counting down the minutes between small bursts of lava from a caldera upslope. While she noticed the weight of exhaustion from the long climb, she said, “I felt really strong.”

Thomas and the 12 other students on the trip visited Stromboli as part of a three-week seminar in southern Italy focused on volcanoes, archaeology and the science of Roman concrete – an exceptionally durable material that may hold insights for future materials that are more sustainable or even suitable for building habitats on Mars.

Offered through Stanford’s Bing Overseas Studies Program, the seminar is an opportunity to draw visceral connections between science and history, and to gain a better understanding of Earth along the way.

Video by Kurt Hickman

Nature’s laboratory

The Neapolitan Province in southern Italy is an ideal place to dive into the science of natural hazards and how they have played into daily life and innovation over thousands of years. Densely populated and peppered with dozens of volcanoes, the region ranks as one of the most hazardous on Earth. The ruins of a Roman harbor and an emperor’s villa can be found offshore, sunken like Atlantis as a result of unrest in Earth’s crust. “Not many places on Earth experience this kind of seismicity and volcanism, while being an ancient town and functioning as a modern society,” Vanorio said. “That’s the beauty of the place.”

Underlying the seminar’s excursions and daily lessons in geophysics, the properties of Roman concrete and 3D modeling from drone images was a larger exercise in finding connections between different fields of study. It’s no accident that students chosen to participate in the seminar represented a wide range of majors, including computer science, physics, classics, chemical engineering and political science.

The ancient Italian city of Pozzuoli was shaped by volcanic activity. (Image credit: Kurt Hickman)

“Not many places on Earth experience this kind of seismicity and volcanism, while being an ancient town and functioning as a modern society.”

—Tiziana Vanorio

Assistant Professor of Geophysics

“There are still scientific questions that we don’t know how to answer,” said Vanorio, who discovered natural processes deep in the subsurface of Campi Flegrei that mirror those in Roman concrete, and has used historical texts to shed light on strengths and the characteristics of both volcanic and engineered materials. “The more we leverage knowledge across different disciplines, the more we can address and solve those problems.”

For Amara McCune, BS ’18, who joined a previous seminar in the region led by Vanorio in 2016, the intermingling of geophysics with dives into the region’s culture proved a powerful mix. “The unique combination of learning about Pompeii, volcanic uplift and Rome while being on-site, hearing from local guides and having archaeological and geological experts point out features of a location made for an incredibly rich learning experience,” she said.

Materials inspired by nature

Romans used concrete made with volcanic ash to build long-lasting structures like the amphitheater in Pozzuoli, Italy. (Image credit: Nora Nieminski)

Nearly all concrete today is based on a recipe developed in the early 1800s, which requires a process that’s heavily carbon-intensive. But ancient Romans invented a different recipe for concrete structures that have survived for millennia. Research now suggests this ancient material and the volcanoes that made its key components may hold clues for more sustainable building materials.

Now pursuing a PhD in physics, McCune said the seminar in southern Italy helped to broaden her thinking about how she might apply her degree. “It made me more open to different fields and eager to learn the history and intricacies of the natural world around us,” she said.

During the most recent trip, darkness fell as the group, giddy in anticipation of the volcano’s powerful eruptions, settled in around Stromboli’s rim. “It explodes violently and without warning – these big, loud bang explosions followed by incandescent ash flying into the air,” explained Dulcie Head, a teaching assistant on the trip and a PhD student in geophysics.

By this time, the students could see the ash swirling around them as more than volcanic dirt. They knew that similar ash had been a key ingredient in construction of the amphitheater, harbor and ancient marketplace in Pozzuoli, and even the Pantheon in Rome, with its massive, unreinforced dome – the largest in the ancient world.

“Pozzuoli is possibly the place where Romans, by looking at nature, were inspired to make an iconic material,” Vanorio said. They developed a recipe for concrete that lasts for thousands of years using volcanic ash, lime, tiny volcanic rocks and water, while modern concrete often crumbles within 50 years.

Atop Stromboli, which scientists carefully monitor for safety, the students also had enough Earth science churning through their heads to see the volcano itself as a natural laboratory. “This volcano is literally producing new rocks as we’re sitting here. It’s throwing them at us,” Head explained. “It’s exciting to see such an active process, where a natural event also produces new materials.”

The group bounced and slid down a path on the volcano’s slopes wearing gas masks to protect their lungs from ash and sand kicked up by their feet. Back at their hotel at the foot of the island, they peeled off their masks and washed away Stromboli’s detritus. Later, the group learned how to calculate the trajectory and velocity of the volcano’s arcing ash projectiles with particle-tracking software.

“This was one way for us to use time-lapse images,” Vanorio said. “I wanted students from Earth science, from the classics, and engineers to learn how to use this tool because we are finding ourselves using these kinds of images more and more – often captured by drone – whether it’s to analyze inaccessible outcrops of rocks or map vast ancient sites or a building.”

What could have seemed like abstract calculations took on greater resonance in light of the group’s up-close encounter with the eruption. “I’ll never forget the bright sparks of the eruption against the dark night,” said Sylvia Choo, ’20, who is majoring in classics and biology. “It was incredible to experience the great force of nature.”

Ancient city

Some 150 miles across the cool Tyrrhenian, within the Campi Flegrei or “Burning Fields” caldera, lies downtown Pozzuoli. In this city best known to many Italians as the birthplace of Sophia Loren, the ruins of a Roman marketplace are a hub for cross-disciplinary connections.

Pozzuoli sits on a restless, Manhattan-sized swath of coast where the rotten-egg smell of sulfur laces the air. Solfatara crater, home of Vulcan, the Roman god of fire, gurgles on the edge of town. And just offshore, sculptures, thermal baths, a villa, bright tiled mosaics and other archaeological ruins rest more than 30 feet below sea level, victims of the caldera’s subsidence.

Students swam through a sunken Roman resort town in the underwater archaeological park of Baiae off the coast of Pozzuoli. (Image credit: Kurt Hickman)

Near Pozzuoli’s modern-day waterfront, three columns stand amid the ruins of the old marketplace, or Macellum. The students knew from their studies on campus in the spring that the marble trio held a 2000-year record of long-term subsidence and brief periods of uplift. So as the columns came into view when the group first walked down from their villa residence, several students exclaimed, “Oh, there they are!”

The market, or Macellum, in Pozzuoli carries a visible record of the restless land’s rise and fall over 2,000 years.

Gathering close to the columns for a lecture from Vanorio while Nieminski’s drone buzzed overhead, they could see bands of tiny holes bored by so-called “stone-eater” mussels – marine mollusks that drilled up and down the columns as the rise and fall of the caldera changed how much of the structures extended above the waterline. “They literally made a mark on history,” Choo said.

Using skills developed in on-campus seminars led by Nieminski, the students were able to analyze history at the Macellum and other sites with a lighter touch. They built 3D models of the marketplace from Nieminski’s drone imagery and manipulated them with software to take measurements and answer scientific questions of their own devising.

Thomas, for example, examined the different materials in the columns to understand how weathering and water pressure from below played out over time. The project, she said, allowed her to weave together knowledge from chemical engineering, physics and math, as well as the geophysics lessons from the seminar. “After this seminar, I am even more convinced that many fields can overlap,” she said.

Restless Earth

Ups and downs are part of the fabric of life in Pozzuoli. In the early 1980s, the ground rose more than 6 feet in just two years, an alarming rate of uplift that reshaped the town, leaving the harbor too shallow for docking and forcing the relocation of schools and shops.

Drones for Geoscience

Scientists pilot a drone back to Earth after capturing images for 3D modeling of a volcano in southern Italy. (Image credit: Kurt Hickman)

Earth scientists have taken to the skies. A combination of advances in powerful, low-cost cameras and sensors, drones to carry them and software to process images has expanded the toolset available to researchers seeking to map and model remote, rugged and hazardous surfaces of the globe.

The rising seabed also triggered enough earthquakes to prompt evacuation of nearly 40,000 people – including Vanorio, then a teenager – for two years beginning in 1982. “Everyone was worried,” she said. “People were expecting an eruption, and we were really concerned about the seismic hazard. The houses were not retrofitted seismically.”

But as seminar students learned through lectures and readings this summer, the episode tipped off Stanford scientists to an unusual toughness in the rock here. Other volcanic calderas, like Yellowstone or the Long Valley, located east of Yosemite National Park, tend to release the energy accumulated from uplift fairly soon through earthquakes. “Those calderas experience uplift and then almost immediately, seismic activity starts,” Vanorio explained. “The rocks deform and then they fracture.”

In Pozzuoli, earthquakes didn’t begin until the Campi Flegrei caldera had deformed by nearly 3 feet. “The question from a rock physics point of view has been, what kind of rocks in the subsurface are able to accommodate such large strain without immediately cracking?” The rock capping this caldera, it turns out, contains fibrous minerals mirroring those in Roman concrete that allow it to stretch and bend before failing under stress.

At the marketplace, Vanorio also pointed out that the durability of Roman concrete can be seen in sections of the ancient walls where the bricks made of tuff – a kind of volcanic rock – eroded away long ago, but the mortar made with volcanic ash and lime still remains. “At the end of the day, these ancient sites are made of Earth materials that degrade and change over time,” Vanorio said. “We can use rock physics to understand those materials and learn to preserve them better.”

Living with natural hazards

The iconic cone of Mount Vesuvius rises less than 15 miles west of Pozzuoli. Its eruption in AD 79 is best known for destroying Pompeii, but the same event also buried Herculaneum, a seaside resort town perched on the complex stratovolcano’s western base.

The students spent a morning walking through the town’s narrow streets, noticing elements of everyday life eerily preserved by the superheated pyroclastic flow of ash, molten rock, mud and gas: wooden chests and cupboards, paintings, the burnt wooden door to a study.

As a classics major, Choo had some ideas about what life was like in Roman times before she embarked on the seminar. “But it was fascinating to see the places they used to live, walk and socialize,” she said.

Students hiked Mount Vesuvius to get a firsthand view of how the volcano shaped the region.

Leaving Herculaneum behind in the afternoon, the group piled into a bus bound for Mount Vesuvius, the source of the town’s destruction. Like Campi Flegrei and Stromboli, Vesuvius sits along a tectonic boundary, where the African plate is sliding underneath the Eurasian plate. A tear in the African plate allows heat to seep through Earth’s crust, gradually melting rock and building up pressure under Vesuvius, which has erupted more than 50 times since AD 79 – most recently in 1944.

From the bus drop-off point, a foot trail led the group up toward the volcano’s rim and then down to a shallow shelf in the crater. At the top, more than 4,000 feet above sea level, the air was cool, and the students could see mountains, rolling hills and cities in the distance. Bright green lizards skittered over the rust-colored rocks, fumaroles streamed sulfur-scented steam and Nieminski’s drone canvassed the crater as the students listened to a local guide describe the inner workings and history of the 400,000-year-old volcano.

Thomas remembers looking down at the sea and clouds. “I felt as if I was on top of the world,” she said. “That was the most elated I’ve ever been in life. It made me feel like I was experiencing to the fullest what life has to offer.”

Stanford Vice Provost and Dean of Research Kathryn Moler wants all research resources to be as readily available as books in a library. This model would enable faculty and students to pursue the most innovative research in flexible, collaborative teams.

Materials inspired by nature

Romans used concrete made with volcanic ash to build long-lasting structures like the amphitheater in Pozzuoli, Italy. (Image credit: Nora Nieminski)

Concrete is the most widely used construction material in the world. The material flows into our modern landscapes in such large quantities that production of its basic ingredient – cement – generates more carbon dioxide emissions each year than any country except China or the United States.

Nearly all concrete today is based on a recipe developed in the early 1800s, which requires a carbon- and energy-intensive process to create lime as a binding material for the rest of the ingredients. But ancient Romans invented a different recipe for concrete structures that have survived for millennia, and research now suggests this ancient material – and the volcanoes that made its key components – may hold clues for more sustainable building materials.

Stanford geophysicist Tiziana Vanorio first got interested in Roman concrete while studying the giant caldera created by a long-slumbering volcano in Pozzuoli, Italy. “Romans used volcanic ash and mixed it with lime to make this iconic material,” she explained. “A natural process in the caldera produces lime and forms concrete-like rocks by mixing it with ash – the same volcanic ash that Romans used.”

That mixing creates rock that is compositionally similar to Roman concrete, but stronger. “Roman concrete is not brittle and it doesn’t allow fluids to enter that could degrade the material, but it does not exhibit high strength,” Vanorio said. “The rocks in the caldera are resilient like Roman concrete, meaning they can withstand a large deformation without fracturing, but they are also strong.”

The wall of a Roman amphitheater in Pozzuoli, Italy shows mortar made with volcanic ash and lime enduring long after bricks eroded away. (Image credit: Tiziana Vanorio)

The challenge now is to take the best qualities of each material. “There is more and more need to make materials that use less energy, produce less CO2 and make use of waste by-products,” Vanorio said. “We want to know how we can build upon the recipe of the Roman concrete to make it stronger, similar to the rocks of the caldera.”

Sulfur turns out to be present in both materials and may be an important link. The idea of using sulfur as a binder for concrete dates back to at least the first lunar mission, as the material is found in moon dust. More recently, growing interest in sending humans to Mars has reinvigorated efforts to develop a waterless concrete based on sulfur-rich Martian soil

But the best sulfur concrete today is brittle. “It’s still better than Roman concrete because it has very high strength, but it’s not as good as the rocks from the caldera,” Vanorio said.

Microscopic structures discovered by Vanorio’s team in both Roman concrete and rocks cored from deep in the caldera may lead the way to a better recipe. “We found a matrix of sulfur-rich fibers intertwined like a finely woven cloth,” she said, crediting PhD student Jackson MacFarlane as a key contributor to the research. The fibers allow materials to handle more strain by slowing the propagation of cracks that can lead to catastrophic failure.

In Pozzuoli, the resilience of the caldera’s rocks is both a curse and blessing. For people who live on and around the caldera, Vanorio said, it would be better to have rocks that do not accumulate so much mechanical energy. “Having a rock that builds up a lot of strain can make it a silent threat,” Vanorio said, as that energy must be released eventually – either through an occasional large earthquake or many little cracks over time. “Fortunately, the fibrous rock seems to tear, releasing energy like small exhales. There are no violent earthquakes – that’s the blessing.”

Materials inspired by nature

Romans used concrete made with volcanic ash to build long-lasting structures like the amphitheater in Pozzuoli, Italy. (Image credit: Nora Nieminski)

Concrete is the most widely used construction material in the world. The material flows into our modern landscapes in such large quantities that production of its basic ingredient – cement – generates more carbon dioxide emissions each year than any country except China or the United States.

Nearly all concrete today is based on a recipe developed in the early 1800s, which requires a carbon- and energy-intensive process to create lime as a binding material for the rest of the ingredients. But ancient Romans invented a different recipe for concrete structures that have survived for millennia, and research now suggests this ancient material – and the volcanoes that made its key components – may hold clues for more sustainable building materials.

Stanford geophysicist Tiziana Vanorio first got interested in Roman concrete while studying the giant caldera created by a long-slumbering volcano in Pozzuoli, Italy. “Romans used volcanic ash and mixed it with lime to make this iconic material,” she explained. “A natural process in the caldera produces lime and forms concrete-like rocks by mixing it with ash – the same volcanic ash that Romans used.”

That mixing creates rock that is compositionally similar to Roman concrete, but stronger. “Roman concrete is not brittle and it doesn’t allow fluids to enter that could degrade the material, but it does not exhibit high strength,” Vanorio said. “The rocks in the caldera are resilient like Roman concrete, meaning they can withstand a large deformation without fracturing, but they are also strong.”

The wall of a Roman amphitheater in Pozzuoli, Italy shows mortar made with volcanic ash and lime enduring long after bricks eroded away. (Image credit: Tiziana Vanorio)

The challenge now is to take the best qualities of each material. “There is more and more need to make materials that use less energy, produce less CO2 and make use of waste by-products,” Vanorio said. “We want to know how we can build upon the recipe of the Roman concrete to make it stronger, similar to the rocks of the caldera.”

Sulfur turns out to be present in both materials and may be an important link. The idea of using sulfur as a binder for concrete dates back to at least the first lunar mission, as the material is found in moon dust. More recently, growing interest in sending humans to Mars has reinvigorated efforts to develop a waterless concrete based on sulfur-rich Martian soil

But the best sulfur concrete today is brittle. “It’s still better than Roman concrete because it has very high strength, but it’s not as good as the rocks from the caldera,” Vanorio said.

Microscopic structures discovered by Vanorio’s team in both Roman concrete and rocks cored from deep in the caldera may lead the way to a better recipe. “We found a matrix of sulfur-rich fibers intertwined like a finely woven cloth,” she said, crediting PhD student Jackson MacFarlane as a key contributor to the research. The fibers allow materials to handle more strain by slowing the propagation of cracks that can lead to catastrophic failure.

In Pozzuoli, the resilience of the caldera’s rocks is both a curse and blessing. For people who live on and around the caldera, Vanorio said, it would be better to have rocks that do not accumulate so much mechanical energy. “Having a rock that builds up a lot of strain can make it a silent threat,” Vanorio said, as that energy must be released eventually – either through an occasional large earthquake or many little cracks over time. “Fortunately, the fibrous rock seems to tear, releasing energy like small exhales. There are no violent earthquakes – that’s the blessing.”

Drones for Earth science

Scientists pilot a drone. (Image credit: Kurt Hickman)

Earth scientists have taken to the skies. A combination of advances in powerful, low-cost cameras and sensors, drones to carry them and software to process images has expanded the toolset available to researchers seeking to map and model remote, rugged and hazardous surfaces of the globe.

Nora Nieminski, a sedimentologist and postdoctoral researcher in Stanford’s School of Earth, Energy & Environmental Scientists (Stanford Earth), was an early adopter of drones for Earth science. “One of the largest challenges we face as field geologists is exposure of rock outcrops,” she said. “How can we actually get up close and measure them, or do whatever we need to do?”

Gaining perspective on steep coastal cliffs, broad expanses of intricately textured shoreline and other sites that in the past might have required laborious on-the-ground surveys or analysis of gritty images captured by plane or boat can now be achieved by drone. “This tool in no way replaces field work,” said Nieminski, an instructor on the 2018 seminar in southern Italy focused on volcanoes, archaeology and Roman concrete. “It just really complements it by allowing us to study these rocks in a much more accurate and efficient way.”

The key to harnessing this technology is a technique known as structure-from-motion photography, or photogrammetry, which makes it relatively easy to convert carefully aligned sets of 2D photographs into 3D models that can be measured, analyzed, manipulated and shared.

“Photogrammetry is as old as modern photography,” said Nieminski, whose role in the program was sponsored by a National Science Foundation CAREER Award for seminar leader Tiziana Vanorio. In its simplest application, photogrammetry can be used to measure the distance between two points on land from a photograph, as long as you know the scale of the image. “With advances in photography and aerial vehicles, it’s now much more widely applicable.”

Students in the summer seminar in southern Italy learned how to use structure-from-motion software before leaving campus, honing their skills in building 3D models using dozens of images of household objects, Stanford’s Lake Lagunita and even a sleeping dog.

In Italy, the students were able to use the same approach to answer a variety of questions, ranging from whether the height of risers in an ancient amphitheater corresponds to what would typically be comfortable for adult males, the preferred viewing group (it does), to approximately how long it would take for the crater atop Mount Vesuvius to fill with magma (60 to 70 years).

The combination of aerial imagery with structure-from-motion software or photogrammetry to produce 3D models offers a powerful research tool with applications far beyond geology. “Archaeologists, architects, mine and landscape engineers, historians – even crime investigators and search-and-rescue operations – they all use it,” Nieminski said. “It is an incredible research tool.”

Drones for Earth science

Scientists pilot a drone. (Image credit: Kurt Hickman)

Earth scientists have taken to the skies. A combination of advances in powerful, low-cost cameras and sensors, drones to carry them and software to process images has expanded the toolset available to researchers seeking to map and model remote, rugged and hazardous surfaces of the globe.

Nora Nieminski, a sedimentologist and postdoctoral researcher in Stanford’s School of Earth, Energy & Environmental Scientists (Stanford Earth), was an early adopter of drones for Earth science. “One of the largest challenges we face as field geologists is exposure of rock outcrops,” she said. “How can we actually get up close and measure them, or do whatever we need to do?”

Gaining perspective on steep coastal cliffs, broad expanses of intricately textured shoreline and other sites that in the past might have required laborious on-the-ground surveys or analysis of gritty images captured by plane or boat can now be achieved by drone. “This tool in no way replaces field work,” said Nieminski, an instructor on the 2018 seminar in southern Italy focused on volcanoes, archaeology and Roman concrete. “It just really complements it by allowing us to study these rocks in a much more accurate and efficient way.”

The key to harnessing this technology is a technique known as structure-from-motion photography, or photogrammetry, which makes it relatively easy to convert carefully aligned sets of 2D photographs into 3D models that can be measured, analyzed, manipulated and shared.

“Photogrammetry is as old as modern photography,” said Nieminski, whose role in the program was sponsored by a National Science Foundation CAREER Award for seminar leader Tiziana Vanorio. In its simplest application, photogrammetry can be used to measure the distance between two points on land from a photograph, as long as you know the scale of the image. “With advances in photography and aerial vehicles, it’s now much more widely applicable.”

Students in the summer seminar in southern Italy learned how to use structure-from-motion software before leaving campus, honing their skills in building 3D models using dozens of images of household objects, Stanford’s Lake Lagunita and even a sleeping dog.

In Italy, the students were able to use the same approach to answer a variety of questions, ranging from whether the height of risers in an ancient amphitheater corresponds to what would typically be comfortable for adult males, the preferred viewing group (it does), to approximately how long it would take for the crater atop Mount Vesuvius to fill with magma (60 to 70 years).

The combination of aerial imagery with structure-from-motion software or photogrammetry to produce 3D models offers a powerful research tool with applications far beyond geology. “Archaeologists, architects, mine and landscape engineers, historians – even crime investigators and search-and-rescue operations – they all use it,” Nieminski said. “It is an incredible research tool.”